Specific recognition between biological molecules plays a fundamental role in most processes in the living cell. The proposed research is a comprehensive X-ray crystallographic study on all structural aspects of molecular recognition interactions involving DNA and RNA, i.e., molecular recognition in nucleic acid-protein interaction using tRNA-protamines and tRNA-peptide complexes as a model system; in nucleic acid-aromatic carcinogens, mutagens and dyes interactions using tRNA as a model compound for nucleic acids; in nucleic acid-metal ion interaction using tRNA-metal ion complexes as a model system; and in nucleic acid-nucleic acid interaction, especially double helix-double helix interactions.